全文获取类型
收费全文 | 26871篇 |
免费 | 514篇 |
国内免费 | 328篇 |
专业分类
测绘学 | 751篇 |
大气科学 | 1905篇 |
地球物理 | 5253篇 |
地质学 | 9310篇 |
海洋学 | 2429篇 |
天文学 | 6507篇 |
综合类 | 54篇 |
自然地理 | 1504篇 |
出版年
2021年 | 217篇 |
2020年 | 270篇 |
2019年 | 323篇 |
2018年 | 638篇 |
2017年 | 631篇 |
2016年 | 771篇 |
2015年 | 447篇 |
2014年 | 743篇 |
2013年 | 1381篇 |
2012年 | 848篇 |
2011年 | 1131篇 |
2010年 | 1033篇 |
2009年 | 1346篇 |
2008年 | 1202篇 |
2007年 | 1196篇 |
2006年 | 1208篇 |
2005年 | 907篇 |
2004年 | 890篇 |
2003年 | 815篇 |
2002年 | 762篇 |
2001年 | 644篇 |
2000年 | 669篇 |
1999年 | 585篇 |
1998年 | 585篇 |
1997年 | 546篇 |
1996年 | 424篇 |
1995年 | 419篇 |
1994年 | 434篇 |
1993年 | 334篇 |
1992年 | 328篇 |
1991年 | 269篇 |
1990年 | 328篇 |
1989年 | 286篇 |
1988年 | 267篇 |
1987年 | 290篇 |
1986年 | 253篇 |
1985年 | 336篇 |
1984年 | 359篇 |
1983年 | 343篇 |
1982年 | 342篇 |
1981年 | 265篇 |
1980年 | 274篇 |
1979年 | 230篇 |
1978年 | 219篇 |
1977年 | 224篇 |
1976年 | 189篇 |
1975年 | 195篇 |
1974年 | 182篇 |
1973年 | 181篇 |
1972年 | 115篇 |
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
521.
A numerical model based upon fundamental principles, a standard (k, ) turbulence closure and a finite element integration technique, is applied to separated flows over hills. Predictions are compared to experimental data from a wind tunnel. Although few-equation turbulence closures have been shown to have obvious deficiencies with respect to comparable flows, the model predicts remarkably accurately, without coefficient adjustments of any kind. Even the turbulent intensity is predicted quite realistically. 相似文献
522.
Darren G. Bos Brian F. Cumming Christine E. Watters John P. Smol 《International Journal of Salt Lake Research》1996,5(1):1-15
Zooplankton collected from vertical net tows were related to the environmental variables from 98 lakes from the Interior Plateau of British Columbia. Canonical correspondence analysis showed that both salinity and ionic composition (pH and Mg) of the lake-water made major and significant contributions to the first two ordination axes (=0.42 and 0.11 respectively,P<0.05). BothArtemia franciscana andMoina hutchinsoni had their highest relative abundance in meso-hypersaline waters. However,Artemia franciscana preferred waters that were higher in Mg and Ca, whileMoina hutchinsoni was found in waters that were lower in Mg and Ca. Similarly, at intermediate salinities,Daphnia pulex and the calanoid copepods preferred waters slightly lower in Mg and Ca, whereasCeriodaphnia laticaudata andSimocephalus spp. were relatively more common in waters higher in Mg and Ca. Because the freshest lakes studied varied much less in ionic composition, the zooplankton in these lakes did not show a preference to ionic composition. As expected, multi-generic groups, such as the calanoid copepods, cyclopoid copepods and nauplii, had wider tolerances to conductivity than groups identified to lower taxonomic levels. Significant weighted-averaging regression and calibration models of conductivity were developed based on zooplankton species composition from the study lakes (r
2=0.56,P<0.05). Samples composed largely of multi-generic taxa yielded the worst estimates of salinity in the reconstruction model. This study suggests that zooplankton community composition may be developed into a useful proxy for paleosalinity reconstruction. 相似文献
523.
Summary Direct measurements of the thermal plasma parameters in the topside ionosphere reveal variations of the plasmasphere boundary in the dusk sector. The ACTIVE satellite's near-polar orbits at altitudes of 500 – 1800 km around winter solstice 1989 were used to study the bulge region of the plasmasphere during intervals with different levels of geomagnetic agitation. The narrow, sharply defined trough in electron concentration corresponding to the plasmapause under quiet conditions situated at L = 6 – 7 moved to lower L-values with increasing geomagnetic activity. This narrow trough can be found in all main ion constituents. During periods of moderate geomagnetic activity, following the onset of a weak magnetic storm, a portion of the plasmaspheric bulge region was separated from the main plasmaspheric body. This can be seen in the outer ionosphere as an inner narrow trough at lower L-value. Troughs in light ions need no longer coincide with this in electron concentration. He+ is the most sensitive constituent reflecting the dusk sector plasmaspheric situation at this altitude.Dedicated to the Memory of Professor Karel P 相似文献
524.
The present paper deals with the specification of bed erosion flux that accounts for the effects of sediment-induced stratification in the water column. Owing to difficulties in measuring the bed shear stress b and the erosive shear strength s, we suggest a series of methods that combine laboratory and numerical experiments. A simplified turbulent transport model that includes these effects helps to quantify b and s. Focusing on soft stratified beds, the present study considers erosion rate formulas of the form =f exp {[Tb-Ts]} where is a model constant (=1 for Gularte's (1978) formula and =1/2 for Parchure's (1984) formula). First, the bed erosive strength profile s(Z) is adjusted by forcing the turbulent transport model with measured erosion rates. Second, three procedures are suggested to determine the erosion rate formula coefficients f and : a global procedure and two different layer-by-layer procedures. Each procedure is applied to an erosion experiment conducted in a rotating annular flume by Villaret and Paulic (1986). The use of the layer-by-layer procedure based on a least squares fitting technique provides a closer fit than the global procedure. The present study points out the complementarity of experimental and numerical approaches and also suggests possible improvements in laboratory test procedures. 相似文献
525.
M. T. McCulloch T. K. Kyser J. D. Woodhead L. Kinsley 《Contributions to Mineralogy and Petrology》1994,115(3):303-312
A comprehensive Sr–Nd–Pb–O isotopic study is reported for rhyolites from the Maroa Volcanic Centre in the Taupo Volcanic Zone (TVZ) of New Zealand. The Sr–Nd isotopic compositions of the rhyolites (87Sr/86Sr=0.705236 to 0.705660 and Nd = 2.0 to 0.2) are intermediate between those of primitive basalts (87Sr/86Sr=0.70387 and Nd = 5.3) and the Torlesse basement (87Sr/86Sr=0.709 and Nd = -4.5). The relatively low mantle-like oxygen isotopic compositions of 18 O = 7 ± 0.5 are consistent with the Nd-Sr isotopic constraints in that they can be accounted for by 15% to 25% crustal contamination of a basaltic parent by relatively 18 O-rich Torlesse metasediment. High precision Pb isotopic analyses of plagioclase separates from the Maroa rhyolites show that they have essentially the same compositions as the Torlesse metasedimentary terrane which is itself distinctive from the Western or Waipapa metasediments. Due to the high concentration of Pb in the Torlesse metasediments (>20 ppm) compared to the basalts (<2 ppm), the Pb isotopic composition of the volcanics may be controlled by relatively small amounts (>10%) of crustal contamination. All these results are shown to be consistent with derivation of the rhyolites by 15% to 25% contamination of relatively primitive basaltic magmas with Torlesse metasedimentary crust, followed by extensive, essentially closed system fractionation of the basalt to a magma of rhyolite composition. It is argued that the processes of assimilation and fractionation are separated in both space and time. The voluminous high silica rhyolites, which make up >97% of the exposed volcanism in the continental margin back-are basin environment of the TVZ, therefore appear to be a product of predominantly new additions to the crust with assimilation-recycling of pre-existing crust being of secondary importance. 相似文献
526.
Keller LP Thomas KL Clayton RN Mayeda TK DeHart JM McKay DS 《Geochimica et cosmochimica acta》1994,58(24):5589-5598
A petrographic, geochemical, and oxygen isotopic study of the Bali CV3 carbonaceous chondrite revealed that the meteorite has undergone extensive deformation and aqueous alteration on its parent body. Deformation textures are common and include flattened chondrules, a well-developed foliation, and the presence of distinctive (100) planar defects in olivine. The occurrence of alteration products associated with the planar defects indicates that the deformation features formed prior to the episode of aqueous alteration. The secondary minerals produced during the alteration event include well-crystallized Mg-rich saponite, framboidal magnetite, and Ca-phosphates. The alteration products are not homogeneously distributed throughout the meteorite, but occur in regions adjacent to relatively unaltered material, such as veins of altered material following the foliation. The alteration assemblage formed under oxidizing conditions at relatively low temperatures (<100 degrees C). Altered regions in Bali have higher Na, Ca, and P contents than unaltered regions which suggests that the fluid phase carried significant dissolved solids. Oxygen isotopic compositions for unaltered regions in Bali fall within the field for other CV3 whole-rocks, however, the oxygen isotopic compositions of the heavily altered material lie in the region for the CM and CR chondrites. The heavy-isotope enrichment of the altered regions in Bali suggest alteration conditions similar to those for the petrographic type-2 carbonaceous chondrites. 相似文献
527.
Dr. T. D. Peterson Dr. S. Esperança Dr. A. N. LeCheminant 《Mineralogy and Petrology》1994,51(2-4):251-276
Summary Early Proterozoic ultrapotassic dikes, lava flows, and pyroclastic rocks of the Christopher Island Formation (CIF) erupted throughout an area 600 × 300 km within the Churchill Province of the Canadian Shield at 1.84 Ga. The rocks range from mafic lamprophyres (mg # 60; SiO2 47–54%, mean K2O/Na2O > 4) with phenocrysts of phlogopite + diopside + apatite ± olivine ± magnetite, to phenocryst-poor felsic rocks and sanidine porphyries (SiO255–69%). All samples have high incompatible element contents and display large depletions of high field strength elements relative to K, Rb, Sr, Ba, and Th. The CIF has geochemical and petrographic characteristics of both minettes and lamproites, but overall most closely resembles young Mediterranean lamproites. Felsic rocks of the CIF were produced by crystal fractionation and crustal contamination of mafic ultrapotassic magma, and include both high-silica lamproites strongly enriched in Zr, U, and Th, and weakly potassic to sodic rocks of trachytic composition. Flows and feeder dikes have relatively homogeneous Nd, 1840 Ma (–6 to –11) but highly variable ES., 1840 Ma (–40 to + 100); samples classified as lamproites have higher average Sr. Dike samples have highly variable present-day Pb isotope compositions, ranging from moderately to strongly nonradiogenic. Geochemical and isotopic data are consistent with contributions from depleted Archean lithospheric mantle, and OIB-type convecting mantle, both metasomatized by subduction-related processes during the Early Proterozoic. The lithospheric mantle probably contained Archean enriched domains as well. Proterozoic enrichment may have accompanied shallow underplating of subducted oceanic lithosphere beneath the Churchill Province during amalgamation of the Laurentian supercontinent. There are strong analogies in isotopic composition, and interpreted source region history, between the CIF and lamproites and minettes of the Wyoming Province and western Greenland, which suggest the existence of a Laurentian ultrapotassic superprovince.
With 7 Figures 相似文献
Geochemie und Entstehung der Proterozoischen ultrapotassischen Gesteine der Churchill Provinz, Kanada
Zusammenfassung Altproterozoische, ultrapotassische Gänge, Lavaströme und pyroklastische Gesteine der Christopher Island Formation (CIF), eruptierten in einem Gebiet von 600 × 300 km in der Churchill Provinz des Kanadischen Schildes vor 1.84 Ga. Die Zusammensetzung dieser Gesteine variiert von mafischen Lamprophyren (mg > 60; SiO2 = 47–54%, durchschnittliches K2O/Na2O > 4) mit Phänokristallent von Phlogopit + Diopsid + Apatit + Olivin + Magnetit, bis zu phänokristallarmen felsischen Gesteinen und Sanidinporphyren (SiO2 = 55–69%). Alle Proben zeigen hohe Gehalte an inkompatiblen Elementen und zeigen beträchtliche Verarmung an high field strength Elementen relativ zu K, Rb, Sr, Ba und Th. Die CIF hat geochemische und petrographische Eigenschaften sowohl von Minetten wie von Lamproiten, aber im allgemeinen ähnelt sie am stärksten jungen mediterranen Lamproiten. Felsische Gesteine der CIF wurden durch Fraktionierung und Krustenkontamination aus mafischen ultrapotassischen Magmen gebildet. Letztere umfassen sowohl siliziumreiche Lamproite, die deutlich an Zr, U und Th angereichert sind und schwach potassische bis sodische Gesteine von trachytischer Zusammensetzung. Lavenergüsse und zufuhrgänge zeigent ein relativ homogenes Nd, 1840 Ma (–6 bis –11) aber ein sehr variables Sr, 1840 Ma (-40 bis + 100); Proben die als Lamproite klassifiziert wurden, zeigent höhere durchschnittliche Sr-Werte. Proben von Gängen haben sehr variable Bleiisotopen-Zusammensetzungen, die von mäßig bis stark nichtradiogen variieren. Geochemische und Isotopendaten weisen auf Beiträge aus verarmtem archaischen lithosphärischen Mantel und aus konvektierendem OIB-Typ Mantel hin, die beide während des Alproterozoikums durch Subduktions-Vorgänge metasomatisiert wurden. Der lithosphärische Mantel enthielt wahrscheinlich auch angereicherte archaische Domänen. Proterozoische Anreicherungsvorgänge dürften seichtes Underplating subduzierter ozeanischer Lithosphäre unter der Churchill Provinz während der Amalgamation des laurentischen Superkontinentes begleitet haben. Es gibt starke Analogien in der Isotopenzusammensetzung und in der interpretierten Geschichte der Ursprungsregion, zwischen den CIF und Lamproiten und Minetten der Wyoming Provinz, und des westlichen Grönland. Diese weisen auf die Existenz einer laurentischen ultrapotassischen Superprovinz hin.
With 7 Figures 相似文献
528.
R. S. Mcmillan T. L. Moore M. L. Perry P. H. Smith 《Astrophysics and Space Science》1994,212(1-2):271-280
We have been measuring changes in the radial velocities (RV's) of solar-type stars to search for gravitational perturbations by planets. We transmit violet starlight through a Fabry-Perot etalon interferometer and sense changes in Doppler shift from changes in the fluxes of light on the slopes of stellar absorption lines. Our data now span 6 years. Our observations of the Sun showed earlier that both our technique and the profiles of solar photospheric violet absorption lines can be stable enough to reveal planetary perturbations. We now carry this validation to the spectra of other near-solar-type stars. Annual averages of our RV's of Draconis and Virginis are stable to ±6 m s–1. The slope of our five-year series of RV's of Bootis A is consistent with the star's well-determined visual astrometric orbit about Bootis B. The Fabry-Perot technique of Doppler shift measurement is fully capable of detecting perturbations due to planets with masses and orbits similar to those of Jupiter.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A. 相似文献
529.
J. J. Quenby T. J Sumner S. D. Sidher S. Immler 《Astrophysics and Space Science》1994,216(1-2):213-217
The shadowing effects of the molecular clouds in the nearby interstellar medium on the soft x-ray background has been investigated, using ROSAT WFC data in conjunction with previous rocket B and C band surveys. Shadowing over a 5° extent occurs only for a few percent of the sky, but the mixed model of the ISM is supported.Detailed modelling of the Draco shadowing region shows little evidence for a multi-temperature, hot ISM component. 相似文献
530.
The factors governing the amounts of CO, O2, and O3 in the martian atmosphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller than observed. Good agreement between models and observations of CO, O2, O3, and the escape flux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constants for the reactions CO + OH and OH + HO2 within their specified uncertainties. Similar revisions have been suggested to resolve discrepancies between models and observations of the terrestrial mesosphere. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atomic hydrogen escape, it is much larger than recent calculations of the exospheric escape rate for oxygen. Weathering of the surface may account for the imbalance. Quantification of the escape rates of oxygen and hydrogen from Mars is a worthwhile objective for an upcoming martian upper atmospheric mission. We also consider the possibility that HOx radicals may be catalytically destroyed on dust grains suspended in the atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much higher than the observed quantity. We feel that there is no need at this time to invoke heterogeneous processes to reconcile models and observations. 相似文献